Power controlling network element

ABSTRACT

A power controlling network element for a communication system includes a detector for measuring radiation power present in components of radiation; element parts, for example a channel control unit (CCU), transponders and other modules, for at least one of: (i) transmitting to the detector at least some components of input radiation received at the elements parts from the system; and (ii) generating one or more components of radiation and emitting the one or more components to the detector; and a field programmable gate array (FPGA) for receiving data from the detector indicative of radiation power in the components of radiation received at the detector, and for controlling radiation power emitted from or transmitted through the aforementioned element parts so that the components of radiation received at the detector are regulated in power to within predetermined power limits. Such an element is by virtue of the detector and the FPGA capable of providing accurately controlled radiation components which are not subject to overshoot or unacceptable drift.

The present invention relates to a power controlling network element foruse in optical communication systems. The invention also relates to amethod of adding a channel to such an element.

Conventional optical communication systems routinely now employwavelength division multiplexing (WDM) where radiation propagating inthe systems is partitioned into a plurality of wavebands, each wavebandassociated with an optical channel of the system. Future designs ofcommunication system will include, for example, 32 channels spaced atwavelength intervals of 0.8 nm; such a spacing corresponds to a channelfrequency separation of approximately 100 GHz for optical radiation ofnominally 1.5 μm wavelength.

When optical radiation propagates within the systems, the opticalradiation propagates through a number of optical components which havenon-uniform spectral responses which tends to accentuate radiation powerassociated with some channels relative to other channels. Suchaccentuation can result in problems in erbium doped fibre amplifiers(EDFAs) which are employed within the conventional systems for boostingradiation levels, for example at repeater nodes within the systems.

A further problem encountered in the communication systems is adding andremoving channels without causing disruption to existing establishedchannels or causing radiation power surges which can interfere withexisting channels being amplified in EDFAs.

EDFAs are inherently non-linear devices; composite radiation comprisinga number of radiation components input into an EDFA where one of thecomponents has an amplitude considerably greater than the othercomponents can cause the EDFA to predominantly use its pumping inputenergy to amplify the component of greater amplitude to the detriment ofthe other channels causing data errors to arise in the other channels.

In order to address these problems, the inventor has devised a powercontrolling network element which is capable of controlling channelradiation amplitude as well as adding and removing channels in acontrolled manner without causing overload effects in optical devicessuch as EDFAs which receive radiation from the network element.

According to a first aspect of the invention, there is provided a powercontrolling network element for a wavelength division multiplex (WDM)communication system in which radiation propagating in the system ispartitioned into a plurality of wavebands, each waveband beingassociated with an optical channel, the element being for controllingthe power of one or more optical channels being transmitted through theelement and for controlling the power of one or more new opticalchannels being added at the element, the element being characterised by:

-   (a) measuring means for measuring power of optical channels received    thereat;-   (b) generating means for:    -   (i) transmitting to the measuring means at least some optical        channels of input radiation received at the generating means        from the system; and for    -   (ii) generating the one or more new optical channels being added        at the element and emitting said one or more new optical        channels to the measuring means; and-   (c) controlling means for receiving data from the measuring means    indicative of power of each optical channel received at the    measuring means, and for controlling the power of each optical    channel;    and wherein said one or more optical channels are generated in a    stepwise incremental manner, thereby reducing power overshoot in    said one or more new optical channels as measured by the measuring    means.

The element provides the advantage that it is capable of adding the oneor more new optical channels (radiation components) without causingoverload or overshoot in the element and the system connecteddown-stream.

The one or more new optical channels are preferably initially added bythe element at a power level insufficient to disrupt existing opticalchannels but sufficient for the measuring means to detect them, therebyenabling the controlling means to determine whether or not said one ormore new optical channels are mistuned (i.e. at a correct waveband).

The controlling means is preferably operable to instruct the generatingmeans to increase radiation power of said one or more new opticalchannels initially in relatively larger incremental power steps and thensubsequently in progressively smaller power steps as the radiation powerof said one or more new optical channels approaches a predeterminedpower level. Use of such graded incremental power steps ensures that theelement adds the one or more new optical channels as promptly aspossible.

When the one or more new optical channels have been added, it isbeneficial for the controlling means to instruct the generating means tomaintain the radiation power level of said one or more new opticalchannels within a error band including the predetermined power level.Control within a error band imposes known limits over which radiationpower is maintained.

Preferably, the controlling means is operable to generate an alarmsignal when it has to instruct the generating means to modify theradiation power of said one or more new optical channels ts cumulativelyby more than the error band. Generation of such an alarm signal enablesthe system to take corrective action if necessary, for examplerequesting the element to switch off the one or more new channels.

Advantageously, the error band is modified to accommodate longer-termageing and drift within the element. Such accommodation of ageingeffects enables the element to be run continuously over long periods,for example years, without element part ageing causing spuriousoperation of the element.

Preferably, the radiation received at the measuring means is a portionof radiation output from the element to the communication system. Bysampling the radiation output from the element, the element is capableof correcting for the wavelength response of all parts affectingradiation propagating in the element.

The element conveniently includes optical amplifying means foramplifying radiation output from the generating means to provide theradiation output from the element, the amplifying means including asubsidiary output for providing the portion of radiation for themeasuring means. Inclusion of the amplifying means enables the elementto compensate for attenuation occurring therein, for example in thegenerating means when it is transmitting optical channels received atthe element from the system up-stream.

Preferably, the controlling means is operable to output instructionsrepetitively to the generating means, the instructions includingidentification means which is incremented when a new command is issued.Such operation provides the advantage that the instructions are sent outrepetitively so that temporary interruption of the instructions does notnecessarily mean that commands are not responded to correctly.

In the element, it is important not to disrupt established opticalchannels when introducing one or more new channels. As a precaution, itis preferable that the controlling means is operable:

-   (a) to monitor radiation received at the measuring means;-   (b) to determine whether or not unintended radiation is present at    wavelengths corresponding to optical channels which the generating    means has not been instructed to transmit or generate; and-   (c) to instruct the generating means not to emit or transmit    radiation at radiation wavelengths corresponding to the optical    channel.

Disrupting established optical channels conveying communication trafficcan appear as system unreliability to system clients and is thereforepreferable avoided.

Advantageously, the generating means includes a channel control unit(CCU) for selectively transmitting one or more optical channels receivedfrom the system at the unit. Such units are highly versatile and enableconsiderable functionality to be thereby built into the element.

In order to render the element physically compact, it is desirable thatthe controlling means is implemented as a field programmable gate array(FPGA). Use of the FPGA enables the element to be reconfigured, ifnecessary, to operate in an alternative manner without having toredesign hardware components of the element.

In a second aspect, the invention provides a method of adding one ormore new optical channels at an element according to the first aspect ofthe invention, the method including the steps of:

-   (a) instructing the generating means to transmit or generate said    one or more new optical channels at a power level insufficient to    disrupt existing optical channels in the element but sufficient for    the measuring means to detect said one or more new optical channels;-   (b) monitoring said one or more new optical channels at the    measuring means and controlling means for determining whether or not    said one or more new optical channels are mistuned;-   (c) when said one or more optical channels are not mistuned,    instructing the generating means to increase radiation power of said    one or more new optical channels in a stepwise incremental manner    until their radiation power is within an error band centred about a    predetermined power limit.

An embodiment of the invention will now be described, by way of exampleonly, with reference to the following diagrams in which:

FIG. 1 is a schematic illustration of a power controlling networkelement according to the invention;

FIG. 2 is a diagram of power leveling control provided by the element inFIG. 1: and

FIG. 3 is a flow chart depicting operation of the invention.

Referring to FIG. 1, there is shown a power controlling network element10 included within a dashed line 15. The element 10 comprises aerbium-doped optical fibre amplifier (EDFA) 20, an optical detector 30,a field programmable gate array (FPGA) 40 connected to an IEEE1394 databus 50, a channel control unit (CCU) 60, an expansion shelf 70, anexpansions area 80 and three transponders 90, 92, 94. The element 10further comprises a first optical fibre waveguide 100 connected fromup-stream communications equipment (not shown) to an optical input portof the CCU 60, a second optical fibre waveguide 110 connected to anoptical output port of the EDFA 20 to provide radiation to down-streamcommunications equipment (not shown), and third and fourth optical fibrewaveguides 120, 130 connected from the shelf 70 and the area 80respectively into a fifth optical fibre waveguide 135 connected from anoptical output port of the CCU 60 to an optical input port of the EDFA20. The element 10 additionally comprises sixth, seventh and eighthoptical fibre waveguides 140, 150, 160 connected from optical outputports of the transponders 90, 92, 94 respectively to couple into thefifth waveguide 135. Lastly, the element 10 includes a ninth opticalfibre waveguide 170 connected from an output tap point O₁ of the EDFA 20to an optical input port O₂ of the detector 20.

The FPGA 40 is electrically connected via its bus 50 to control inputsof the CCU 60, to modules in the shelf 70 and the expansions area 80,and to the three transponders 90, 92, 94.

The CCU 60 is a proprietary module manufactured by MarconiCommunications Ltd. and includes a subsystem manufactured by a vendor inthe USA. The CCU 60 comprises free-path optics, optical filters, and aliquid crystal matrix of windows which can be directed to selectivelyattenuate radiation received thereat and transmitted therethrough. Thefilters are operable to separate radiation received at the CCU 60 fromthe fibre waveguide 100 into a plurality of rays, each ray correspondingto one of the 32 channels of the element 10. Each window of the matrixis capable of receiving its associated ray and thereby selectivelytransmitting and attenuating radiation corresponding to its associatedchannel to the fibre waveguide 135. The CCU 60 exhibits a minimuminsertion loss of 6 dB and radiation of each channel propagatingtherethrough can be attenuated in a range of 6 to 36 dB. The matrix hasassociated therewith control electronic circuits which interface to thebus 50 or directly to firmware (not shown).

Operation of the network element 10 will now be described in overview.The element 10 is operable to provide power leveling control of up to 32communication channels accommodated by the element 10, the channelsbeing at wavelength spacings of substantially 100 GHz. “Leveling” hererefers to controlling the radiation power of radiation corresponding toeach of the channels to within predetermined limits. The element 10 iscontrolled by firmware (not shown in FIG. 1) which communicates eithervia the FPGA 40 or directly with the CCU 60, the transponders 90, 92, 94and the modules included in the expansion shelf 70 and the expansionsarea 80. The firmware instructs them regarding which communicationchannels they can use when outputting radiation, and manages when theycan start and terminate transmitting communication traffic on aparticular channel allocated thereto. The firmware attempts to preventtwo or more of the modules, transponders or CCU from operating onidentical channels simultaneously which would result in radiationinterference in the EDFA 20 and subsequent loss of information relatingto such channels.

Radiation of channels transmitted through the CCU 60, and radiationoutput from the transponders 90, 92, 94 and the modules of the expansionshelf 70 and of the expansion area 80 is coupled as composite radiationinto the fibre 135 and propagates to the EDFA 20. The EDFA 20 amplifiesthe composite radiation and outputs amplified composite radiation at itsoutput port into the fibre waveguide 110. A portion of the amplifiedcomposite radiation is coupled to the output tap point O₁ wherefrom theportion propagates through the fibre waveguide 170 to the optical inputport O₂ of the detector 30. The detector 30 is operable to measure theradiation power of radiation input at the Port O₂ and to output datadescribing radiation power in each of the channels to the FPGA 40. Undercontrol from the firmware, the FPGA 40 instructs the CCU 60, thetransponders 90, 92, 94, and the modules in the expansion shelf 70 andthe expansion area 80 regarding radiation wavelength and power levelthat they are permitted to transmit or emit radiation; the FPGA 40 sendsits instructions out via the bus 50.

The FPGA 40 thus provides power control which ensures that radiationclashes do not occur in the EDFA 20, and that the channels haveassociated therewith radiation of mutually similar radiation power whenin use so that the EDFA 40 and similar EDFAs connected down-stream tothe fibre waveguide 110 are not overloaded. The FPGA 40 is also operableto check whether or not the transponders 90, 92, 94 and the modules aremistuned when they commence transmitting at the wavelength of any of thechannels indicated by the FPGA 40.

When performing its leveling function, the FPGA 40 communicates to theCCU 60, the transponders 90, 92, 94 and the modules at an interval of 1ms. The FPGA 40 sends repetitively a stream of 32 bytes of command dataas provided in Table 1; Channel 1 data is sent at the start of thestream, Channel 2 data thereafter and so on until Channel 32 data at theend of the stream.

TABLE 1 Channel 1 Channel 2 . . . and so on until . . . Channel Channeldata data 31 data 32 data

Thus, the stream comprises 32 bytes in total and repeats every 1 ms.Each byte is structured as provided in Table 2 (located at the end ofthe description) and includes bit 0 to bit 7 where the bit 7 is aleading bit and the bit 0 is an end bit of each byte.

As each byte is sent from the FPGA 40 using isocronous data packets,there is no confirmation back to the FPGA 40 that the byte reaches itsintended destination. For example, loss of data sent from the FPGA 40can occur if the bus 50 is for any reason reset.

End bit 0 and bit 1 are used to ensure that data commands are executedat the CCU 60, the transponders 90, 92, 94 and the modules. The bits 0,1 form a binary value ID where bit 1 is more significant than bit 0.Whenever a new command is sent from the FPGA 40, the ID value isincremented by 1 count. When the ID value reaches a binary value 11, itis incremented on the next command to a binary value 00, namely the IDvalue is incremented in a modulo-4 manner. Thus, the stream is repeatedon a 1 ms basis with similar ID value until the FPGA 40 updates thestream with a new command when it increments the value of ID. Suchupdating corresponding to a new command being issued from the FPGA 40and is performed at a time interval in a range of 1 ms to 1 second. Therepetitive stream is received at the CCU 60, the transponders 90, 92, 94and the modules which monitor the stream and identify when a new commandis issued by the FPGA 40 and will store the command and act upon it asappropriate.

Thus, if the CCU 60, the transponders 90, 92, 94 and the modules shouldmiss several streams, for example 10 consecutive streams, during aninterruption of the bus 50, it will not receive new ID bits. When theinterruption has ended, the CCU 60, the transponders 90, 92, 94 and themodules will again receive new ID values and then respond to the newcommand as appropriate.

Bit 2 is used by the FPGA 40 to indicate whether a byte in the streamrelates to the transponders 90, 92, 94 or to the CCU 60; a value 0 forbit 2 corresponds to the CCU 60 whereas a value 1 corresponds to thetransponders 90, 92, 94 or the aforementioned modules.

Bits 3 to 6 determine the command itself and concern an incrementalchange in emitted radiation power or an absolute emitted power setting,for example “channel off”. Incremental changes conveyed in the datastream are graded relatively in size, namely a tiny step being at afirst low size limit and a huge step at a second high size limit. Small,medium, large, extra large (X large) are intermediate step sizes insequence from the first limit to the second limit. A null, namely bits6-3 being 0000 respectively, is transmitted when correction to emittedor transmitted radiation power output is not required. However, a“channel off” command, namely bits 6-3 being 0001 respectively, istransmitted when a channel is to be turned off. When “null” and “channeloff” commands are issued, bit 7 is set to a zero value to avoid thecommands as being interpreted as “absolute power” commands.

Bit 7 determines whether a byte in the stream corresponds to aninstruction for incremental increase in emitted radiation power orsetting the CCU 60, the transponders 90, 92, 94 or the modules to anabsolute level of emitted radiation power.

As described in the foregoing, the FPGA 40 is operable to monitorradiation power in channels output from the EDFA 20. The FPGA 40functions by monitoring whether or not its most recent commands issuedvia the bus 50 have been responded to. For example, if the FPGA 40issues a command via the bus 50 to the CCU 60 to change radiation powertransmission therethrough for a specific channel by 1 dB, the FPGA 40will monitor the portion of composite radiation input to the detector 30for up to 4 repeats of the stream to check that the change has beenimplemented; if at least 50% of the change has been implemented withinthese four repetitions, the command is deemed to have been responded tocorrectly.

The CCU 60, the transponders 90, 92, 94 and the modules are alsooperable to monitor the stream received thereat from the bus 50. Ifchanges in the ID value do not occur, then the CCU 60, the transponders90, 92, 94 and the modules function to interpret such a lack of changesas failure of the FPGA 40 or its associated bus 50.

Now operation of the network element 10 will be described when it addsor subtracts a channel. When addition occurs of a channel to the networkelement 10 where the element 10 presently has allocated in it less than32 channels, there being a maximum allowable limit of 32 channels asdescribed in the foregoing, a method is adopted to operate the element10 so that disruption of existing established channels does not arise.

Addition of a new channel requires interaction between the firmware andthe FPGA 40 regarding which part of the element 10 is to outputradiation corresponding to the new channel. The firmware checks via thebus 50 that the part of the element 10, for example the transponder 90,is presently receiving “channel off” commands in the stream from theFPGA 40. If this is not so, it is likely that the part is presentlyconveying communication traffic and that resetting it would disruptestablished communication traffic passing through the element 10.

Once the FPGA 40 confirms to the firmware that the new channel can beadded, the firmware sends a series of messages to the part of theelement 10 via the EEEE1394 bus instructing it to proceed to add the newchannel. There then follows a controlled sequence of commands where theFPGA 40 controls an incremental increase of output radiation powercorresponding to the new channel; the sequence of commands ensures thatthe new channel is increased promptly in power to a predetermined limitwithout overshooting in power or providing a sudden burst of radiationpower to the EDFA 20 disrupting its operation and corruptingcommunication traffic presently propagating therethrough. The sequenceof commands comprises the following steps:

Step 1:

The FPGA 40 sends a command via the stream conveyed through the bus 50to the part of the element 10 to set it to an absolute level (bit 3=1).The part responds by emitting radiation at a wavelength corresponding tothe new channel but at a low radiation power level, “low” correspondingto just sufficiently large for the detector 30 to detect and providenon-zero power measurements to the FPGA 40 but insufficiently powerfulto disrupt established channels if the part is mistuned and generatesoutput radiation at a wavelength not corresponding to the new channel.“Low” in the context of the element 10 is in the order of −25 dBm.

Step 2:

The FPGA 40 then checks that sending commands to the part has resultedin an increase in power corresponding to the new channel. Such a checkis necessary because, if the part is mistuned, further increases inpower could disrupt existing established channels in the element 10.

Step 3:

When the FPGA 40 has established that the part is emitting radiation ata wavelength corresponding to the new channel, the FPGA 40 issuescommands via the bus 50 to the part, starting initially with relativelylarger step sizes, for example huge or X large steps, and thenprogressively using smaller step sizes, for example small steps, toincrease the radiation power output from the part as detected by thedetector 20 for the new channel; the commands are issued by the FPGA 40until the radiation power of the new channel approaches a predeterminedtarget power level. Such a graduated increase in power provides abenefit that overshoot in radiation power for the new channel issubstantially avoided; overshoot would inevitably occur if the FPGA 40were to send commands only specifying huge or large steps.

Step 4:

When the part is emitting radiation amplified by the EDFA 20 to withinone small step of the target power level, the FPGA 40 switches to acontrolling mode of operation where a relatively narrow 1 dB limit isset for a degree that the part can be instructed to reduce or increaseits radiation output. In the controlling mode, radiation power in thenew channel is no more than a small step size away the target powerlevel. Each active channel of the network 10 has associated therewith aninternal power drift register in the FPGA 40; a register for the newchannel is set to zero value and the new channel is allowed to driftregarding its radiation output power level. For every active channelwithin the network element 10, the FPGA 40 accumulates a record of allthe step sizes it issues in commands via the bus 50; for example, if thefollowing steps were issued in commands: +0.02 dB, +0.02 dB, +0.02 dB,−0.08 dB, +0.04 dB, a register recording these commands would contain avalue +0.02 dB. If the radiation power level is somewhat low for the newchannel, and a step up is required to be issued from the FPGA 40 torestore the new channel to within the aforementioned 1 dB limits, theFPGA 40 checks the power drift register for the new channel to establishwhether or not it would overflow if the step up were issued as acommand. If overflow were to occur, a step size would then be selectedby the FPGA 40 that would not cause the power drift register to exceedthe aforementioned 1 dB limits. A channel control out of range alarm israised by the FPGA 40 when one or more of the power drift registers isat or beyond its 1 dB limit. Such a feature prevents the FPGA 40 fromissuing excessive step up commands via the bus 50 to compensate fortemporary attenuation effects, for example temporary attenuation arisingon account of a bent or broken fibre waveguide. However, the FPGA 40 isalso designed to enable it to compensate for anticipated power driftover element 10 part lifetimes; in order to achieve such long-termcompensation, the power drift registers are periodically checked todetermine if they contain values of 0.75 dB or more. If one or more ofthem do contain such a value, the values of these one or more registersare reduced by 0.01 dB, thereby causing the one or more registers todrift back to zero over a longer time scale. Such drift towards zerovalue enables the FPGA 40 to accommodate any slow attenuation or powerdrift within the element 10 and communication parts connected thereto;“slow” in this context means a rate of less than 0.01 dB/second.

Step 5:

The FPGA 40 maintains radiation power on a per channel basis not onlyfor the new channel but also for existing established channels of theelement 10. Although the stream is repetitively transmitted via the bus50 each 1 ms, different parts of the element 10 exhibit mutuallydifferent response times. For example, the transponders 90, 92, 94 arecapable of responding within a few tens of milliseconds whereas the CCU60 can require up to 200 ms to respond on account of time taken for theopacity of the matrix to alter under electronic control. The FPGA 40 isdesigned to cope with these different response times.

Steps 1 to 5 above describe the sequence.

After the new channel has been added and subsequently level controlled,the FPGA 40 records details regarding the channel and the part providingits radiation. Such recording avoids the FPGA 40 over a longer periodproviding instructions to increase radiation power cumulatively to morethan 1 dB although, as described above, long term drift greater than 1dB is accommodated by FPGA 40; this recording protects the EDFA 20 frombeing overloaded over a relatively shorter time period. An examplescenario will now be described:

Radiation for the new channel is provided by a module in the expansionshelf 70; the new channel is being controlled in the controlling mode.The fibre waveguide 120 is then disturbed causing an increase inattenuation. The FPGA 40 then attempts to correct for a decrease inradiation associated with the new channel detected at the detector 30 byincreasing radiation output power of the module. A few minutes later,the waveguide 120 is disturbed again and attenuation in the waveguide120 reduces back to its original level before being disturbed. The powerin the new channel received by the EDFA 20 will now be 1 dB which canresult in amplification for other channels propagating through the EDFA20 being consequently reduced due to power hogging within the EDFA 20.The FPGA 40 can in such circumstances raise an alarm and issueinstructions via the bus 50 for the module providing radiation of thenew channel to be shut down and cease emitting radiation at thewavelength associated with the new channel.

It is for avoiding severe hogging effects under fault conditions thatthe aforementioned limits have been set at a value of 1 dB for shorterterm drift.

When the new channel is to be removed from the element 10, the firmwaresends a message to the module via the bus 50 including “channel off”commands to which the module responds.

For safety purposes, in an event of a fibre connected to the element 10becoming broken, for example the fibre 100, an ALS command is issued tothe FPGA 40 which immediately issues commands via the bus 50 to allparts of the element 10 to cease emitting radiation or transmittingradiation.

The EDFA 20 and the detector 30 are separately temperature controlled.In particular, the EDFA 20 is temperature controlled so that it cannotoperate at relatively high temperatures where its operating lifetime isreduced.

The detector 30 must be operating within a temperature range where itprovides accurate measurement of radiation power output from the EDFA 20for each of the channels, otherwise there is a risk that the element 10can cause overload in optical communication equipment down-stream. Theelement 10 is operable to communicate with the firmware that it is intemperature calibration, in other words in “temperature lock”. If theelement 10 subsequently drifts out of temperature lock, it raises analarm because it can no longer guarantee its power leveling functionsfor its channels. The FPGA 40 in such a situation will issue nullcommands in the stream to all parts of the element 10 capable ofgenerating or transmitting channel radiation; such operation is referredas a “free run” mode. The firmware or an operator can then decide toshut down the element 10 or allow it to continue in its “free run” mode.

The FPGA 40 is operable to monitor radiation of channels output at theEDFA 20 which are nominally supposed to be switched off. If the FPGA 40in combination with the detector 30 detects radiation corresponding toone or more of these switched off channels, the FPGA 40 sends an alarmto the firmware that the one or more channels should not be used.

The FPGA 40 is also operable to monitor radiation of channels output atthe EDFA 20 and issue commands to switch off channels whose radiationpower exceeds a predetermined power level.

The FPGA also includes digital averaging for filtering noise and datatraffic power fluctuations associated with measuring radiation power atthe output of the EDFA 20. When the FPGA 40 is adding a new channel,such averaging has associated therewith a shorter time constant relativeto averaging performed when the new channel is under leveling controlwithin its 1 dB limits. Such changing of the time constants enables theelement 10 to respond more rapidly when channel radiation power levelsare supposed to be changing, for example when a new channel is added.

In order to further elucidate operation of the element 10 with regard toSTEPs 1 to 5 above, reference will now be made to FIG. 2 which is agraph of power leveling control provided by the element 10 illustratedin FIG. 1; the graph is indicated by 300 and includes an abscissa axis310 denoting time and an ordinate axis 320 denoting radiation outputpower in the new channel as measured at the output of the EDFA 20. Alongthe axis 310, there are four time points marked, namely t₀, t₁, t₂, t₃where the point to precedes the point t₃ in time.

During a time interval between the points t₀ and t₁, the firmware checksthat a part of the element 10 identified for providing radiation for thenew channel is presently receiving “channel off” commands. During a timeinterval between the points t₁ and t₂, the part outputs radiation at alevel of −25 dBm which is insufficiently powerful to disrupt establishedchannels within the element 10, but sufficiently powerful to be justdetected by the detector 30 in cooperation with the FPGA 40; the FPGA 40checks that the radiation is output at a wavelength corresponding to thenew channel. In a time interval between the points t₂ and t₃, the FPGA40 issues incremental power increase commands to the part, for examplestarting with huge steps and progressively reducing step sizes to largeand then to small for avoiding overshoot. After the time point t₃, theelement 10 reverts to leveling control operation where a target power ofPT is to be maintained within 1 dB power limits.

It will be appreciated that modification can be made to the networkelement 10 without departing from the scope of the invention. Forexample, the step sizes corresponding to bits 6 to 3 can be modifiedfrom those provided in Table 2. Moreover, the 1 dB power limit forleveling control can be in a range 0.5 to 2 dB when the element 10 isdesigned to be more tolerant to radiation power variations; however, therange can be extended further if the element 10 and its associatedsystem can tolerate higher degrees of overload. Although, only threetransponder 90, 92, 94 are illustrated in FIG. 1, the element 10 canincorporate other numbers of transponders. The expansion shelf 70 andthe expansions area 80 can be equipped with a variety of different typesof modules for outputting channel radiation. Moreover, the element 10can be operated so that it adds more than one new channelsimultaneously.

TABLE 2 Most significant Least significant bit 7 Bit 6 Bit 5 Bit 4 Bit 3Bit 2 Bit 1 bit 0 Power Step size Channel destination MessageIdentification 0 => incremental mode 0 0 0 0 (Null) 0 => CCU 60 0 0 (ID0) 0 0 0 1 (Channel off) 1 (ID 1) 1 => absolute mode 0 0 1 0 (+Tinystep) 1 => transponders 0 0 (ID 2) 0 0 1 1 (+Small step) 90, 92, 94 1(ID 3) 0 1 0 0 (+Medium step) 1 0 1 0 1 (+Large step) 0 1 1 0 (+X largestep) 1 0 1 1 1 (+Huge step) 1 0 0 0 (−Tiny step) 1 0 0 1 (−Small step)1 0 1 0 (−Medium step) 1 0 1 1 (−Large step) 1 1 0 0 (−X large step) 1 10 1 (−Huge step) 1 1 1 0 (Spare) 1 1 1 1 (Spare)

1. A power controlling network element for a wavelength divisionmultiplex (WDM) communication system in which radiation propagating inthe system is partitioned into a plurality of wavebands, each wavebandbeing associated with an optical channel, the element being operativefor controlling a power of one or more optical channels beingtransmitted through the element and for controlling a power of one ormore optical channels being added at the element, the elementcomprising: a) measuring means for measuring the power of each opticalchannel output from the element, the power of a new optical channelbeing newly added at, or newly transmitted through, the element beinginitially at a level insufficient to disrupt existing optical channels;b) a channel control unit for selectively transmitting to an output ofthe element the one or more optical channels of input radiation receivedfrom the system, and for individually controlling the power of said oneor more optical channels transmitted to the output; c) generating meansfor generating and controlling the power of the one or more opticalchannels being added at the element, and for emitting said one or moreoptical channels to the output; and d) controlling means for receivingdata from the measuring means indicative of the power of each opticalchannel output from the element, and for controlling the power of eachoptical channel to within predetermined power limits by increasing thepower in a stepwise incremental manner, thereby reducing power overshootin said one or more optical channels, wherein the controlling means isoperable for monitoring data corresponding to the radiation received atthe measuring means, for determining whether or not unintended radiationis present at radiation wavelengths corresponding to optical channelswhich at least one of the channel control unit and the generating meanshas not been instructed to respectively transmit or generate, and forinstructing the at least one channel control unit and the generatingmeans not to transmit or emit radiation at radiation wavelengthscorresponding to the unintended optical channel.
 2. A power controllingnetwork element for a wavelength division multiplex (WDM) communicationsystem in which radiation propagating in the system is partitioned intoa plurality of wavebands, each waveband being associated with an opticalchannel, the element being operative for controlling a power of one ormore optical channels being transmitted through the element and forcontrolling a power of one or more optical channels being added at theelement, the element comprising: a) measuring means for measuring thepower of each optical channel output from the element, the power of anew optical channel being newly added at, or newly transmitted through,the element being initially at a level insufficient to disrupt existingoptical channels; b) a channel control unit for selectively transmittingto an output of the element the one or more optical channels of inputradiation received from the system, and for individually controlling thepower of said one or more optical channels transmitted to the output; c)generating means for generating and controlling the power of the one ormore optical channels being added at the element, and for emitting saidone or more optical channels to the output; and d) controlling means forreceiving data from the measuring means indicative of the power of eachoptical channel output from the element, and for controlling the powerof each optical channel to within predetermined power limits byincreasing the power in a stepwise incremental manner, thereby reducingpower overshoot in said one or more optical channels, wherein when anoptical channel is to be newly added at, or newly transmitted through,the element, the controlling means is operable for instructing thegenerating means to initially generate the new optical channel at apower level sufficient for the measuring means to detect butinsufficient to disrupt existing optical channels transmitted orgenerated by the element.
 3. The element according to claim 2, whereinthe new optical channel is initially at a power level sufficient for themeasuring means to detect; wherein the measuring means is operable tomeasure a wavelength of the new optical channel; and wherein thecontrolling means is operable to determine whether the new opticalchannel is mistuned and, when the new optical channel is not mistuned,the controlling means is operable to instruct at least one of thechannel control unit and the generating means to increase the radiationpower of the new channel in a stepwise incremental manner.
 4. Theelement according to claim 3, wherein the controlling means is operableto instruct the at least one of the channel control unit and thegenerating means to increase the radiation power of the new opticalchannel initially in relatively larger incremental power steps, and thensubsequently in progressively smaller power steps as the radiation powerof the new optical channel approaches a predetermined power level. 5.The element according to claim 4, wherein the controlling means isoperable to instruct the at least one of the channel control unit andthe generating means to maintain the radiation power level of the newoptical channel within an error band including the predetermined powerlevel.
 6. The element according to claim 5, wherein the controllingmeans is operable to generate an alarm signal when the controlling meansinstructs the at least one of the channel control unit and thegenerating means to modify the radiation power of the new opticalchannel cumulatively by more than the error band.
 7. The elementaccording to claim 6, wherein the error band is modified to accommodatelonger-term ageing and drift within the element.
 8. The elementaccording to claim 2, wherein the radiation received at the measuringmeans is a portion of radiation output from the element to thecommunication system.
 9. The element according to claim 8, includingoptical amplifying means for amplifying the radiation output from thechannel control unit and the generating means to provide the radiationoutput from the element, the amplifying means including a subsidiaryoutput for providing the portion of radiation for the measuring means.10. The element according to claim 2, wherein the controlling means isoperable to output instructions repetitively to the channel control unitand the generating means, the instructions including identificationmeans which is incremented when a new command is issued.
 11. The elementaccording to claim 2, wherein the controlling means is implemented as afield programmable gate array (FPGA).